Once NX viewer is launched and closed, Double clicking on a part file causes the part gets opened only in NX viewer not in NX application.
Solution Start -> Run- > regedit
Set the following value data to (Default) Value Name according to your installation path.
"C:\apps\NX12\NXBIN\ugs_router.exe" -ug -use_file_dir "%1" -version=V30.0 -opts –nx
Now the part should be opened in NX instead of viewer.
Hardware/Software Configuration Platform: INTL64
OS Version: 1064